1. Field of the Invention
This invention relates to a color image display system of the color look-up table type, and more particularly to such a system which displays an image originally having many more color components than the number of entries of the color look-up table, or colors that can be displayed simultaneously, without substantial deterioration of image quality.
2. Description of the Prior Art
Recently, color image display systems for processing color images have begun to employ color look-up tables. FIG. 1 shows an example of such a prior art color image display system, wherein image data for one frame is digitally stored in a refresh memory 1 with a bit map system, said memory 1 being accessed by the vertical and the horizontal synchronization systems of a CRT (cathode ray tube) 2 to sequentially address a color look-up table 3. The look-up table 3 feeds digital signals representing the desired primary colors of red, green and blue to D/A (digital-to-analog) converters 4R, 4G, and 4B in a later stage according to the addressing. Then, based on the outputs of the D/A converters the CRT 2 is driven.
In such a color image display system, the number of colors that may be displayed on the screen of CRT 2 is determined by the number of stages of the D/A converters 4R, 4G and 4B. For example, if each of the converters 4R, 4G and 4B has eight stages, 16,777,216 (=256*3) colors may be displayed. On the other hand, the number of colors that can be simultaneously displayed on the screen of CRT 2 is determined by the number of entries of the color look-up table 3, or the number of bits of pixel data that are stored in the refresh memory 1. For example, if the pixel data are of eight bits, only 256 (=2*8) colors can be displayed simultaneously. Usually, while the number of colors that may be displayed can be designed to be large by increasing the number of stages of each D/A converter, still the number of colors that can be displayed simultaneously may be small due to the reduced number of bits of the pixel data.
Of course, it may be possible to increase the number of bits in the pixel data, and to employ, for the color look-up table 3, one which has a very large number of entries. This arrangement would allow an original image to be stored in the refresh memory 1 without substantial deterioration in its quality, and permit it to be faithfully displayed on the CRT 2. For example, in the case where a digitizer is used that generates eight bits for each of the components, red, green and blue of a single pixel, no deterioration in the image quality would occur if the pixel data is stored in the refresh buffer 1 with 24 bits, and is subsequently decoded by the color look-up table 3 that has entries of 16,777,216 (=2*24).
However, such an arrangement would be of little practicality in view of the processing speed and complexity of its construction. Thus, it is conventionally arranged to improve performance without deterioration of image quality by suitably selecting the simultaneously displayed colors, and then suitably mapping colors composing an original image or original colors to said selected simultaneously displayed colors. Such selection of the simultaneously displayed colors and the mapping are called color quantization.
The popularity algorithm and the median cut algorithm are known techniques for this quantization. These algorithms first determine the colors to be simultaneously displayed, and map the original colors to the simultaneously displayed colors. The popularity algorithm refers to the popularity of particular color elements used to determine the simultaneously displayed colors. FIG. 2 will help to illustrate the technique used to recognize the popularity. FIG. 2 shows a color space that is formed by color data in which each pixel or pel of an image has eight bits assigned for each of its red, green and blue color content. Each color data is represented by an element (i,j,k) in the color space (where i, j and k have values ranging from 0 to 255). Each of, for example, 1,048,576 (=1024.times.1024) pixels in a frame image has its color data distributed to the elements (i,j,k) in the color space by scanning the image with a digitizer. The number of pixels divided to each element is called popularity of elements. A correlation table between each element and its popularity is called a three-dimensional histogram. The popularity algorithm determines the colors to be simultaneously displayed according to the size of popularity. For example, in the case where the number of the simultaneously displayed colors is 256, the 256 color elements having the largest popularity are selected as the simultaneously displayed colors.
The median cut algorithm utilizes each color in a color map to represent an equal number of pixels in an original image, wherein the color space is sequentially divided into two so that each of two regions contains equal numbers of pixels. Finally, it divides the color space into the same number of regions as the simultaneously displayed colors, for example, 256 regions, and then determines the colors representing these divided regions respectively, which become the simultaneously displayed colors.
Reference may be had to "SIGGRAPH'82 Proceedings Vol. 16, No. 3, July, 1982, p. 297-307 [ACM]" for more details of the popularity algorithm and the median cut algorithm.
Although the popularity algorithm and the median cut algorithm were proposed for color quantization as above, these algorithms have shortcomings in that they often cause errors when the simultaneously displayed colors are selected, and they have complicated procedures to map the original colors to the simultaneously displayed colors.